Gantry comprising beam analyser for use in particle therapy

ABSTRACT

The present invention relates to a particle therapy apparatus used for radiation therapy. More particularly, this invention relates to a gantry for delivering particle beams which comprises means to analyze the incoming beam. Means are integrated into the gantry to limit the momentum spread of the beam and/or the emittance of the beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityto U.S. application Ser. No. 13/502,946, filed Apr. 19, 2012 (pending),which is a continuation of PCT Application PCT/EP2010/065707, filed Oct.19, 2010, which claims priority to European Application EP09173989.6,filed Oct. 23, 2009. The contents of the above-referenced applicationsare expressly incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a charged particle therapy apparatusused for radiation therapy. More particularly, this invention relates toa rotatable gantry designed for receiving a charged particle beam in adirection substantially along a rotation axis of the gantry, fortransporting and for delivering said beam to a target to be treated.

STATE OF THE ART

Radiotherapy using charged particles (e.g. protons, carbon ions, . . . )has proven to be a precise and conformal radiation therapy techniquewhere a high dose to a target volume can be delivered while minimizingthe dose to surrounding healthy tissues. In general, a particle therapyapparatus comprises an accelerator producing energetic chargedparticles, a beam transport system for guiding the particle beam to oneor more treatment rooms and, for each treatment room, a particle beamdelivery system. One can distinguish between two types of beam deliverysystems, fixed beam delivery systems delivering the beam to the targetfrom a fixed irradiation direction and rotating beam delivery systemscapable of delivering beam to the target from multiple irradiationdirections. Such a rotating beam delivery system is further namedgantry. The target is generally positioned at a fixed position definedby the crossing of the rotation axis of the gantry and the centraltreatment beam axis. This crossing point is called isocenter andgantries of this type capable of delivering beams from variousdirections to the isocenter are called isocentric gantries.

The gantry beam delivery system comprises devices for shaping the beamto match the target. There are two major techniques used in particlebeam therapy to shape the beam: the more common passive scatteringtechniques and the more advanced dynamic radiation techniques. Anexample of a dynamic radiation technique is the so-called pencil beamscanning (PBS) technique. In PBS a narrow pencil beam is magneticallyscanned across a plane orthogonal to the central beam axis. Lateralconformity in the target volume is obtained by adequate control of thescanning magnets. Depth conformity in the target volume is obtained byadequate control of the beam energy. In this way, a particle radiationdose can be delivered to the entire 3D target volume.

The particle beam energies required to have sufficient penetration depthin the patient depend on the type of particles used. For example, forproton therapy, proton beam energies are typically ranging between 70MeV and 250 MeV. For each required penetration depth the beam energyneeds to be varied. The energy spread of the beam should be limited asthis directly influences the so-called distal dose fall-off.

However, not all accelerator types can vary the energy. For fixed energyaccelerators (e.g. a fixed isochronous cyclotron) typically an energyselection system (ESS) is installed between the exit of the acceleratorand the treatment room as shown in FIGS. 1, 2 and 3. Such an energyselection system is described by Jongen et al. in “The proton therapysystem for the NPTC: equipment description and progress report”, Nuc.Instr. Meth. In Phys. Res. B 113 (1996) 522-525. The function of theEnergy Selection System (ESS) is to transform the fixed energy beamextracted from the cyclotron (e.g. 230 MeV or 250 MeV for protons) intoa beam having an energy variable between the cyclotron fixed energy downto a required minimum energy (for example 70 MeV for protons). Theresulting beam must have a verified and controlled absolute energy,energy spread and emittance.

The first element of the ESS is a carbon energy degrader which allows todegrade the energy by putting carbon elements of a given thicknessacross the beam line. Such an energy degrader is described in patentEP1145605. As a result of this energy degradation, there is an increasein emittance and energy spread of the beam. The degrader is followed byemittance slits to limit the beam emittance and by a momentum or energyanalysing and selection device to restore (i.e. to limit) the energyspread in the beam.

A layout of such a known energy selection system 10 is shown in FIG. 1together with a stationary, fixed energy accelerator 40 (in this examplea cyclotron). After the degrader and emittance limiting slits, the beampasses through a 120° achromatic bend made up of two groups of two 30°bends. To meet the specification for the distal fall off, the momentumspread or the energy spread in the beam is limited by a slit placed atthe center of the bend. The beam is focused by quadrupoles before thebend and between the two groups of two 30° bending magnets so that theemittance width of the beam is small and the dispersion is large at theposition of the slit.

The entire beam line starting at the energy degrader 41 up to thetreatment isocenter 50 forms an optical system that is achromatic, i.e.a beam-optical system which has imaging properties independent frommomentum (dispersionless) and independent from its transverse position.The beam line can be divided in multiple sections and each section isforming itself an achromat. As shown in FIG. 2, the first section is theESS 10 followed by an achromatic beamline section that brings the beamup to the entrance point of a treatment room. In the case of a gantrytreatment room, this entrance point is the entrance point or couplingpoint of the rotating gantry 15. The gantry beam line is then forming athird achromatic beam line section. In the case of a single treatmentroom particle therapy configuration, as shown in FIG. 3, the beam linecomprises two achromatic beam line sections: a first section is the ESS10 that brings the beam up to the gantry entrance point and the secondachromatic section corresponds to the rotating gantry 15 beam line. Atthe gantry entrance point, the beam must have the same emittance in Xand Y in order to have a gantry beam optics solution that is independentfrom the gantry rotation angle. The X and Y axis are perpendicular toeach other and to the central beam trajectory. The X axis is in thebending plane of the dipole magnets.

A disadvantage of the use of such a degrader and energy analyser is thatthis device requires a relative large space area as shown in FIG. 1 andhence a large building footprint is required. The installation of an ESSresults also in an extra equipment cost.

The present invention aims to provide a solution to overcome at leastpartially the problems of the prior art. It is an objective of thepresent invention to provide a charged particle therapy apparatus thathas a reduced size and that can be built at a reduced cost when comparedto the prior art particle therapy apparatus.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized by the appendedclaims.

In the prior art particle therapy configurations as shown for example inFIGS. 1 to 3, the functionalities of limiting the momentum spread (orenergy spread, which is equivalent) and the emittance of the beam isperformed by a separate device, namely with the energy selection system(ESS) 10, which is installed between the stationary accelerator 40 andthe rotating gantry 15. As shown on FIG. 1, a first element of the ESSis an energy degrader 41 which is used to degrade the energy of theparticle beam of the fixed-energy accelerator 40.

With the present invention, a rotatable gantry beam delivery system isprovided having a gantry beam line configuration which fulfills multiplefunctions:

-   -   The known function of transporting, bending and shaping an        entering particle beam in such a way that a particle treatment        beam can be delivered at a gantry treatment isocenter for use in        particle therapy;    -   The additional function of limiting the energy spread of the        entering particle beam to a selected maximum value.

With the present invention, the ESS functionality of limiting the energyspread or momentum spread of the beam to a selected value is performedby the gantry system itself. Hence the size and cost of a particletherapy facility can be reduced.

In the context of the present invention, the momentum spread is definedas the standard deviation of the momentums of the particles at a givenlocation and is expressed as a percentage of the average momentum of allparticles at this location. Whatever the location of the means forlimiting the momentum spread in the gantry, these means are preferablydesigned for limiting said momentum spread to 10%, more preferably to5%, and even more preferably to 1% of the average momentum of allparticles.

Preferably, the gantry also fulfills a second additional function oflimiting the transverse beam emittance of the entering particle beam toa selected maximum value, which further reduces cost and size of theparticle therapy facility.

More preferably, the gantry according to the invention also comprises acollimator installed in-between the gantry entrance point and a firstquadrupole magnet in the gantry. This collimator is used for reducingthe emittance of the beam before the beam is arriving at the firstmagnet in the gantry beam line.

In an alternative preferred embodiment, the above mentioned collimatoris installed outside of the gantry, i.e. in-between the energy degraderand the entrance point of the gantry.

According to the invention, a particle therapy apparatus is alsoprovided comprising a stationary particle accelerator, an energydegrader and a rotatable gantry having means to limit the momentumspread of the beam. Preferably said gantry also comprises means to limitthe emittance of the beam.

Alternatively, a particle therapy apparatus is provided comprising astationary particle accelerator, an energy degrader, a rotatable gantrycomprising means to limit the momentum spread of the beam and acollimator installed in-between said energy degrader and said gantry forlimiting the emittance of the beam. More preferably, said gantrycomprises additional means to limit the emittance of the beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representation of a known energy selection system for usewith a fixed energy cyclotron

FIG. 2 shows a typical layout of a known particle therapy beam lineconfiguration

FIG. 3 shows a schematic representation of a known layout of a singleroom particle therapy configuration

FIG. 4 shows schematically an exemplary embodiment of a device accordingto the invention.

FIG. 5 shows results of beam optics calculation for an exemplary gantryaccording to the invention.

FIG. 6 shows results of beam optics calculation for another exemplarygantry configuration according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described in detail in relation to theappended drawings. However, it is evident that a person skilled in theart may conceive several equivalent embodiments or other ways ofexecuting the present invention. The drawings described are onlyschematic and are non-limiting. In the drawings, the size of some of theelements may be exaggerated and not drawn on scale for illustrativepurposes.

A exemplary particle therapy configuration according to the invention isshown in FIG. 4. In this example, the rotatable gantry according to theinvention is coupled with a stationary-, fixed energy-particleaccelerator 40 to form a single room particle therapy apparatus 100. Anexample of a particle accelerator for protons is a superconductingsynchrocyclotron which has a compact geometry (e.g. with an extractionradius of 1.2 m). The gantry according to the invention is installed inthe gantry room and a shielding wall (e.g. a 1.7 m thick concrete wall)separates the gantry room from the accelerator room. An energy degrader41 is installed between the accelerator 40 and a gantry entrance point45 (coupling point). This energy degrader 41 is positioned within theaccelerator room just in front of the shielding wall 52 separating theaccelerator room from the gantry room. The gantry entrance point 45 islocated after the degrader 41 and is an entrance window for the beamline of the gantry. This entrance window 45 is the first part of agantry beam line section where the beam is entering the gantry in adirection substantially along the rotation axis of the gantry. Therotation axis of the gantry is indicated by a horizontal dash-dottedline passing through the isocentrer 50 and the entrance point 45. Asshown in FIG. 4, there is no momentum or energy analyser deviceinstalled between the degrader and the gantry entrance point as is thecase in the prior art systems (FIGS. 1 to 3).

Similar as in the prior art configurations shown in FIGS. 1 to 3, thereis a short beam line section between the exit of the accelerator and thedegrader 41, where for example two quadrupole magnets 44 are installedfor transporting and focussing the beam into a small spot (for examplebetween 0.5 mm and 2 mm one sigma) at the energy degrader. The energydegrader 41 is for example a rapidly adjustable, servo controlled,rotating, variable thickness, cylinder of degrading material (asdisclosed in EP1145605). The distance between the exit of theaccelerator and the degrader can be about 2 m. Other types of energydegrading systems, e.g. lateral moving wedge shaped based degraders canbe used as well.

The energy degrader currently used by the applicant has at its entrancean integrated horizontal-vertical beam profile monitor which allowsmeasurement of the size and position of the beam spot and, through acontrol system algorithm, means for automatic tuning of the up-streambeam optics. Hence, the beam at the degrader 41 can be well defined, forexample, the beam is focused into a small waist with a half width notexceeding 2 mm in both planes. With these input beam conditions, theoutput emittance of the beam degraded in energy is dominated by multiplescattering in the degrader and is relatively independent from the inputconditions. The resulting beam after energy degradation can beconsidered as a diverging beam from a virtual waist in X and Y at thedegrader with a given size and divergence. The two orthogonal coordinateaxis X and Y are perpendicular (transverse) to the central beamtrajectory. The emittances in X and Y (also called “transverseemittances”) can be considered to be substantially identical at thispoint. The larger the energy reduction introduced by the degrader, thelarger will be the transversal emittance in X and Y and the larger willbe the momentum spread of the degraded beam.

The embodiment of the invention is a gantry configuration comprisingmeans 43 to limit the momentum spread of the incoming beam. A beamentering the gantry comprising particles having an average momentumvalue and a momentum spread.

To limit the momentum spread of the incoming beam, a pair of momentumanalysing slits 43 are installed in the gantry.

These momentum analysing slits 43 are preferably located at a positionalong the beam path where the particles of the beam are dispersedaccording to their momentum.

More preferably, these slits are installed at a position where thenominal dispersion is larger than the nominal beam size. The nominaldispersion is defined as a transversal displacement of a particle whosemomentum differs by 1% (one percent) of an average momentum P of allparticles of the beam. The nominal beam size is defined as the one sigmabeam size value in X of a mono-energetic particle beam having theaverage momentum P. Suppose that the nominal dispersion is 2.5 cm: thismeans that a particle having a momentum P′=1.01·P will be displaced by2.5 cm in X from a particle having momentum P. In this example, aparticle having a momentum P′=0.99·P will also be displaced in X by 2.5cm but having an X coordinate with an opposite sign.

The momentum limiting slits can for example be installed at a positionwhere the nominal beam size in X is between 0.2 cm and 1 cm and thenominal dispersion in X is between 1 cm and 3 cm. By opening or closingthe slits, the maximum momentum spread that is required (selected) canbe obtained. One can for example select to limit the maximum momentumspread to 0.5% of the average momentum by adjusting the slitscorrespondingly. If one wants to limit the maximum momentum spread to0.4% of the average momentum, then one has to close the pair of momentumslits more. For this purpose a calibration curve can be established,defining the slit opening as function of the required momentum spread.

In the configuration of FIG. 4, the nominal dispersion is large incomparison with the beam size at a position in-between gantry quadrupolemagnet number seven and the second dipole magnet 48 and hence this is apreferred position to install the momentum spread limiting slits. Theseslits can for example be installed just before the second dipole magnet48. The exact position can vary depending on the detailed gantryconfiguration.

Instead of using a pair of slits as means for reducing the momentumspread of the beam, other means can be used as well. For example one canuse apertures or collimators with various diameters which can be put inthe beam line, preferably at the above discussed positions.

In the example shown in FIG. 4, a gantry for delivering scanning beamsat the treatment isocenter 50 is presented and the beam line of thisgantry comprises three dipole magnets 47,48,49 and seven quadrupolemagnets 44. In this gantry configuration, scanning magnets 46 areinstalled upstream of the last dipole magnet 49. Between the gantryentrance point 45 and the first dipole magnet and in between the firstand second dipole magnet there are respectively, two and five quadrupolemagnets.

Preferably, in addition to the means 43 to limit the momentum spread ofthe beam, also means 42 to limit the transverse beam emittance can beinstalled in the gantry 15. For this purpose, two pairs of slits (in Xand Y) limiting the beam divergence can for example be installedin-between the second quadrupole magnet and the first dipole magnet 47.Hence, by limiting the divergence of the beam, the transverse beamemittance, which is proportional to the beam divergence, is limited. Thefirst two quadrupoles installed in the gantry in-between the entrancepoint 45 and the first dipole magnet 47 serve to focus the divergentbeam, originating from the degrader, before the beam reaches thedivergence limiting slits. To what extent the beam emittance needs to bereduced will depend on what the maximum emittance the gantry can acceptto efficiently transport the beam and it will also depend on what thebeam requirements are at the treatment isocenter (such as for examplethe beam size required at the treatment isocenter). Acceptable beamemittances and beam sizes may depend on the technique used for shapingthe beam (e.g. pencil beam scanning or passive scattering). The examplegiven in FIG. 4 is for a scanning beam delivery system. For a pencilbeam proton scanning system the beam emittance can for example belimited to 7.5 Pi mm mrad in both X and Y. For practical beam tuningpurposes, just in front, downstream, of the divergence limiting oremittance limiting slits, a beam profile monitor can be installed (notshown on FIG. 4). Instead of using a pair of slits in X and Y as meansfor reducing the divergence of the beam, other means can be used aswell. For example one can use apertures or collimators with variousdiameters which can be put in the beam line.

If the energy reduction of the beam is very large (e.g. reduction of 250MeV protons down to 70 MeV), the emittance and divergence of the beambecomes very large and the diameter of the beam, just before the firstquadrupole magnet in the gantry, can become larger than the diameter ofthe beam line pipe. For this purpose a collimator (not shown in FIG. 4)can furthermore be installed upstream of the first quadrupole magnet inthe gantry 15 to cut off already a part of the beam. This collimator canbe installed in the gantry 15 in-between the entrance point 45 and thefirst quadrupole magnet of the gantry. Alternatively, such a collimatorcan be installed outside the gantry, i.e. in-between the degrader andthe entrance point 45 of the gantry 15. When such a collimator forlimiting the emittance of the beam is installed in either of the twopositions mentioned above, in an alternative gantry embodiment the means42 for limiting the emittance can be omitted.

When a particle beam hits divergence and/or momentum limiting slits,neutrons are produced. To limit the neutron radiation at the level ofthe treatment isocenter 50 where the patient is positioned, adequateshielding need to be provided. As neutrons are mainly emitted in thedirection of the beam, one can install just after the first dipolemagnet, across the axis of rotation of the gantry, a neutron shieldingplug 51 to shield the neutrons produced on means to limit the emittanceof the beam installed upstream of the first dipole magnet 47. As theneutrons are mainly emitted in the direction of the beam, neutronsproduced at the momentum limiting slits 43 are not directing to thepatient. Nevertheless, a local neutron shielding (not shown on FIG. 4)can be installed around the momentum limiting slits 43 in order toreduce overall neutron background radiation.

In order not to overload FIG. 4, details of the mechanical constructionof the gantry have been omitted on purpose. Examples of such mechanicalelements not shown on FIG. 4 are: two spherical roller bearings forrotating the gantry by at least 180° around the patient, a gantry driveand braking system, a drum structure for supporting a cable spool, acounterweight needed to get the gantry balanced in rotation, etc.

When designing a gantry for particle therapy, several beam opticalconditions need to be fulfilled. At the gantry entrance point 45 thebeam must have identical emittance parameters in X and Y in order tohave a gantry beam optics solution that is independent from the gantryrotation angle. As discussed above, these conditions are naturallyfulfilled when placing the energy degrader just in front of the gantryentrance point. In addition, the following beam optical conditions needto be met:

-   -   1. The gantry beam-optical system must be double achromatic,        i.e. the beam imaging properties must be independent from        momentum (dispersionless) and independent from position.    -   2. The maximum size of the beam (one sigma) inside the        quadrupoles should preferably not exceed 2 cm in order to keep a        reasonable transmission efficiency in the gantry.        There is also a third condition that however can vary depending        on the technique used for shaping the beam as discussed above.        For a scanning system this third condition can be described as        follows:    -   3. At the isocenter 50 the beam must have a small waist, of        substantially identical size in X and Y.        For a scattering system, required beam sizes can be specified        more upstream of the isocenter (for example at the exit of the        last bending magnet) and the acceptable beam sizes for        scattering are in general larger than for scanning (for example        1 cm at the exit of the last bending magnet).        In addition to these three conditions (1 to 3), new requirements        are introduced resulting from the current invention:    -   4. At the position of the energy spread limiting slits 43, the        nominal dispersion in X should preferably be large in comparison        with the nominal beam size in X (for examples of values see        discussion above).        Preferably, a gantry according to the invention also comprises        means to limit the emittance of the beam. This results in an        additional requirement:    -   5. At the position of the emittance limiting slits 42, the beam        must have beam optical parameters (size and divergence) in X and        Y that allow to cut the divergence. This means for example that        the beam must have a reasonable size (e.g. 0.5 cm to 2 cm, one        sigma).

The gantry configuration shown in FIG. 4 is based on a beam opticalstudy performed with the beam optics “TRANSPORT” code (PSI GraphicTransport Framework by U. Rohrer based on a CERN-SLAC-FERMILAB versionby K. L. Brown et al.). The beam envelopes in X and Y in the gantry beamline for an entering proton beam of 170 MeV are shown in FIG. 5 as anexample. The beam envelopes are plotted for the X direction and Ydirection in the lower panel and upper panel, respectively. In thisexample the emittance of the final beam is 12.5 Pi mm mrad. Thiscorresponds to a situation where the divergence of the incoming beam hasbeen limited to 6 mrad in X and Y. The beam transported through thesystem can then be considered as a beam starting at the degrader with asmall beam spot of 1.25 mm and a divergence of 6 mrad. With this beamoptics a beam size at the treatment isocenter of 3.2 mm (one sigmavalue) is obtained which is an adequate value for performing pencil beamscanning. The positions of the quadrupole magnets and dipole magnets areshown on FIG. 5. The transversal positions of the dipole magnets (thevertical gaps) are not shown on scale in this figure and the purpose isonly to indicate their position along the central trajectory. Especiallythe gap in X an Y of the last bending magnet 49 are much larger than onthe scale of FIG. 5 as a large opening is needed because the scanningmagnets are positioned upstream of this dipole magnet and a largescanning area need to be covered at isocenter. The position of thescanning magnets along the beam path is indicated by a vertical line.The dotted line represents the nominal dispersion in X of the beam. Asshown, just before the second dipole magnet 48 a large nominaldispersion value is obtained and this is the position where the momentumlimiting slits 43 are preferably installed. The position along thecentral beam trajectory of the momentum limiting slits 43 is indicatedby a vertical line on FIG. 5. The nominal beam size in X at the momentumlimiting slits is about 0.23 cm while the nominal dispersion in X atthis position is about 2.56 cm, hence obtaining a good momentumseparation of the incoming beam. Preferably, also divergence limitingslits 42 are used. A good position for these slits 42 is indicated onFIG. 5 by a vertical line. At this position, the beam size in X and Y isabout 1.8 cm and 0.6 cm, respectively. This beam optical solutionpresented fulfills the conditions of a double achromat.

In the example shown in FIG. 4 and FIG. 5, a three dipole gantryconfiguration was used with dipole bending angles of respectively 36°,66° and 60°. However, the invention is not limited to a specific gantryconfiguration for what concerns number of dipoles or bending angles ofthe dipoles. The invention is neither limited to the number ofquadrupole magnets and the relative positions of the quadrupoles withrespect to the dipole magnets.

As a second example, the invention has been applied to a conical twodipole large throw gantry. This corresponds to the gantry configurationshown on FIG. 2 and FIG. 3. These large throw gantries have been builtby the applicant and are discussed by Pavlovic in “Beam-optics study ofthe gantry beam delivery system for light-ion cancer therapy”, Nucl.Instr. Meth. In Phys. Res. A 399(1997) on page 440. In these gantries afirst 45° dipole magnet bends the beam away from the axis of rotation ofthe gantry and the beam then further follows a second straight beam linesection before entering the second 135° dipole magnet which is bendingand directing the beam essentially perpendicular to the axis ofrotation. The straight beam line section between the gantry entrancepoint and the first 45° dipole magnet comprises, in the original gantrydesign, four quadrupole magnets (FIG. 2 is a configuration having onlytwo quadrupole magnets installed in this beam line section), and thesecond straight section between the first and second dipole magnetcomprises five quadrupole magnets. With this gantry the distance betweenthe exit of the last bending magnet and the treatment isocenter is 3 mand the beam shaping elements configured in a so-called nozzle areinstalled upstream of the last bending magnet. This nozzle uses eitherthe passive scattering technique or the scanning technique for shapingthe beam conform the treatment target. The scanning magnets are part ofthe nozzle and are hence installed downstream of the last gantry dipolemagnet.

A beam optical analysis has been performed for this two dipole gantryconfiguration. The same conditions and requirements as discussed abovehave been respected. The resulting beam envelopes in this gantry areshown in FIG. 6 for a proton beam of 160 MeV. The beam envelopes areplotted for the X direction and Y direction in the lower panel and upperpanel, respectively. The positions along the central beam path of the45° dipole magnet 67, the 135° dipole magnet 68 and the variousquadrupole magnets 44 are indicated in FIG. 6. Also here the energydegrader is installed just before the entrance window of the gantry and,as an example, in this calculation the divergence was cut at 8 mrad andthe emittance of the final beam is 10 Pi mm mrad both in X and Y. Thebeam envelope as shown in FIG. 6 starts at the gantry entrance windowand the beam has a size of 1.25 mm (one sigma value). In this gantryconfiguration the first straight section between the entrance window andthe first 45° gantry bending magnet 67, comprises four quadrupolemagnets 44. Divergence limiting devices 42 are installed in between thesecond and third quadrupole magnet and are indicated by a vertical lineon FIG. 6. The momentum spread limiting slits 43 are installed at aposition where the nominal dispersion in X is large compared to thenominal beam size. The dotted line on FIG. 6 represents the nominaldispersion in X of the beam. The position of the momentum spreadlimiting slits 43 are indicated by a vertical line on FIG. 6. At thisposition the nominal dispersion is about 2.6 cm in X and the nominalbeam size in X (one sigma value) is about 0.6 cm which is adequate foranalysing the incoming beam according to momentum and limiting themomentum spread to a given value by setting the slits at thecorresponding position. The beam envelope shown in FIG. 6 is a tuningsolution for a nozzle using the scanning technique (the scanning magnetsare installed downstream of the 135° dipole magnet but are not shown onFIG. 6). This gantry configuration used in this beam optics study alsocomprises two quadrupole magnets installed upstream of the 135° lastdipole magnet 68 as indicated on FIG. 6. With this tuning solution, adouble waist in X and Y is obtained at isocenter having a beam size of 4mm (one sigma value), which is suitable for performing pencil beamscanning. This beam optical solution fulfills the conditions of a doubleachromat.

A particle therapy apparatus 100 can be formed by combining astationary, fixed energy particle accelerator, an energy degrader and arotatable gantry according to the invention, i.e. a rotatable gantrycomprising means for limiting the energy spread or momentum spread ofthe beam and preferably also comprising means for limiting the emittanceof the beam. As shown on FIG. 4, which is an example of a proton therapyapparatus, a compact geometry can be obtained and the building footprintthat is needed to install this apparatus is smaller than with a separateenergy selection system.

Although the embodiments described are focussing on proton gantries, theinvention is not limited to proton gantries. The person skilled in theart can easily apply the elements of the invention, i.e. means foranalysing the beam (limiting the emittance and limiting the energyspread), to gantries for use with any type of charged particles such ase.g. a gantry for carbon ions or other light ions.

Gantries for particle therapy have been designed since many years and,in combination with stationary, fixed energy particle accelerators, aseparate energy selection system was always installed in the beam linebetween the accelerator and the gantry. According to the presentinvention a new gantry configuration is provided comprising means forlimiting the energy spread or momentum spread of the beam and preferablyalso comprising means for limiting the emittance of the beam. Hence thegantry itself comprises functionalities of the standard prior art energyselection system. By designing a gantry with these means to analyse thebeam as described, a more compact particle therapy apparatus can bebuilt.

The invention claimed is:
 1. A particle therapy apparatus comprising: aparticle beam generator configured to output a particle beam having afirst energy; an energy degrader for reducing an energy of the particlebeam from the first energy to a second energy, such that the energyspread of the particle beam output by the energy degrader is increasedwith respect to an energy spread of the particle beam output by theparticle beam generator; and a rotatable gantry configured to receivethe particle beam having the second energy in a direction substantiallyalong a rotation axis of the rotatable gantry, the rotatable gantryincluding: a beam optic system having a plurality of dipole magnetsconfigured for transporting the particle beam along a beam path to atarget, wherein the beam optic system includes, with respect to the beampath, a first dipole magnet and a last dipole magnet, wherein the beamoptic system is configured to have a first position along the beam pathwhere a nominal dispersion at the first position is larger than anominal beam size at the first position, and wherein the first positionis located downstream of the first dipole magnet and upstream of thelast dipole magnet; and a spread-limiting device, installed at the firstposition, to limit an energy spread of the particle beam having thesecond energy, such that the limited energy spread is decreased, withrespect to the energy spread of the particle beam having the secondenergy, to a selected maximum energy spread.
 2. The particle therapyapparatus according to claim 1, wherein the spread-limiting deviceincludes a spread-limiting aperture.
 3. The particle therapy apparatusaccording to claim 1, wherein the spread-limiting device includes acollimator.
 4. The particle therapy apparatus according to claim 1,wherein the particle beam generator is a cyclotron.
 5. The particletherapy apparatus according to claim 1, wherein the first energy of theparticle beam is an energy of 250 MeV.
 6. The particle therapy apparatusaccording to claim 1, wherein the particle beam is a proton beam, andthe second energy is between 70 MeV and 250 MeV.
 7. The particle therapyapparatus according to claim 1, wherein the rotatable gantry isconfigured to rotate at least 180 degrees.
 8. The particle therapyapparatus according to claim 1, wherein a controller defines thesettings of the plurality of dipole magnets of the beam optic system. 9.The particle therapy apparatus according to claim 8, wherein thecontroller is configured to adjust magnetic strengths of one or more ofthe plurality of magnets of the beam optic system such that the particlebeam having the second energy has the nominal dispersion larger than thenominal beam size at the first position.
 10. The particle therapyapparatus according to claim 9, wherein the spread-limiting device hasan opening through which at least a portion of the beam passes through,and wherein the controller is configured to control the spread-limitingdevice based, at least in part, on a calibration curve defining theopening of the spread-limiting device as a function of a momentum of theparticle beam.
 11. The particle therapy apparatus according to claim 1,where the beam dispersion is between 1 cm and 3 cm and the beam size isbetween 0.2 cm and 1 cm at the first position.
 12. The particle therapyapparatus according to claim 1, wherein the first dipole magnet has abending angle of 45° and the last dipole magnet a has a bending angle of135°.
 13. The particle therapy apparatus according to claim 1, wherein:the nominal dispersion is a transversal displacement of a particlehaving momentum differing by 1% of an average momentum of all particlesof the beam, and the nominal beam size being is a one sigma beam sizevalue of a mono-energetic particle beam having the average momentum. 14.A particle therapy apparatus comprising: a particle beam generatorconfigured to output a particle beam having a first energy; an energydegrader for reducing an energy of the particle beam from the firstenergy to a second energy, such that an energy spread of the particlebeam output by the energy degrader is increased with respect to anenergy spread of the particle beam output by the particle beamgenerator; and a rotatable gantry configured to receive the particlebeam having the second energy in a direction substantially along arotation axis of the gantry, and wherein the gantry includes: aplurality of dipole magnets including a first dipole magnet and a lastdipole magnet configured to transport the particle beam having thesecond energy along a beam path to a target, and a spread-limitingstructure located in the beam path at a first position where a nominaldispersion of the particle beam having the second energy is larger thana nominal beam size of the particle beam having the second energy andhaving a spacing to limit an energy spread of the particle beam havingthe second energy, such that the limited energy spread is decreased,with respect to the energy spread of the particle beam having the secondenergy, to a selected maximum energy spread value, wherein the firstposition is located downstream of the first dipole magnet and upstreamof the last dipole magnet.
 15. The particle therapy apparatus accordingto claim 14, wherein the first dipole magnet has a bending angle of 45°and the last dipole magnet has a bending angle of 135°.
 16. The particletherapy apparatus according to claim 14, wherein: the nominal dispersionis a transversal displacement of a particle having momentum differing by1% of an average momentum of all particles of the beam, and the nominalbeam size is a one sigma beam size value of a mono-energetic particlebeam having the average momentum.
 17. The particle therapy apparatusaccording to claim 14, wherein the spread-limiting structure includes acollimator.
 18. The particle therapy apparatus according to claim 14,wherein the spread-limiting structure is part of the gantry.
 19. Theparticle therapy apparatus according to claim 14, wherein the particlebeam generator is a cyclotron.
 20. The particle therapy apparatusaccording to claim 14, wherein the first energy of the particle beam isan energy of 250 MeV.
 21. The particle therapy apparatus according toclaim 14, wherein the particle beam is a proton beam, and the firstenergy is between 70 MeV and 250 MeV.
 22. The particle therapy apparatusaccording to claim 14, wherein the rotatable gantry is configured torotate at least 180 degrees.
 23. The particle therapy apparatusaccording to claim 14, wherein a controller defines the settings of theplurality of dipole magnets of the beam optic system.
 24. The particletherapy apparatus according to claim 23, wherein the controller isconfigured to adjust magnetic strengths of one or more of the pluralityof magnets of the beam optic system such that the particle beam havingthe second energy has the nominal dispersion larger than the nominalbeam size at the first position.
 25. The particle therapy apparatusaccording to claim 14, where the beam dispersion is between 1 cm and 3cm and the beam size is between 0.2 cm and 1 cm at the first position.